This invention relates to weighing apparatus and more particularly to a modular "smart" digital load cell and the use of a rocker pin as a load responsive spring element, or counterforce.
The so-called "rocker pin" is a column with curved end surfaces and has been used in weighing applications for a number of years, usually as a load transmitting device. A principal advantage of the rocker pin is that it can be made self-erecting, that is, so that when the normally upright pin is deflected about its base or grounded end the pin will return to its upright position when the deflecting load is removed. The self-erecting feature is an advantage in weighing applications in which temporary side loads are encountered. The self-erecting feature is obtained by configuring the pin so that the radius of curvature of each end surface is greater than half the total height of the pin. The rocker pin is also simple to manufacture in that all shaping required is circular and can be accomplished by turning the pin on a lathe. No drilling, tapping or machining is required. The pin is, therefore, inexpensive to manufacture.
Columnar structures have been provided with strain gages or similar transducers and employed as counterforces in weighing applications. The performance of the column in this respect has not been entirely satisfactory, however, primarily because of nonlinearity problems. The column when loaded yields unequal tensile and compressive strain which produces a nonlinear strain characteristic as compared to counterforces which yield nearly equal tensile and compressive strains. Some linearity correction has been obtained by addition of a semiconductor strain gauge in series with the input voltage to the strain gauge bridge which varies the voltage of the bridge in such a way as to compensate for the nonlinearity. Nonlinearity remains, however, a substantial disadvantage of the columnar load cell.
Recently there has appeared the so-called "digital load cell" in which an analog-to-digital converter and microprocessor are dedicated to a single load cell. The electronic circuits are mounted on a printed circuit board connected directly to the counterforce. This development has permitted digital correction of various load cell inaccuracies.